1. Field of Invention
The present invention relates to a jitter measurement circuit and an integrated circuit applying the same. More particularly, the present invention relates to a jitter measurement circuit built-in an ASIC, for measuring jitter value of a clock generated from a clock generator in the ASIC.
2. Description of Related Art
In integrated circuits, such as data transmission circuits, PLL (Phase Locked Loop) is widely used for clock generation. For instance, PLL circuits are utilized in many applications to provide an output signal that is of the same phase and frequency as an input reference signal. In addition, PLL are widely used in Application Specific Integrated Circuit (ASIC) chips for clock synchronization and multiplication to facilitate high speed chip to chip communication. However, PLL have some small amount of error associated with their operation. Because of the high degree of precision required by many of today's advanced digital systems, jitter parameters of output clocks or reference clocks generated from PLL or CDR (Clock Data Recovery) circuits are important for circuit designers and manufacturers. The clock signals should be prevented from being affected by clock skews due to jitters. In particular, when the frequencies of the clock signals are high, the influences of the jitters to these clock signals should be monitored and prevented so that the relevant integrated circuit can be normally operated.
PLL or CDR produces some amount of undesirable error (or jitter) in its output clock whose magnitude is usually small and difficult to measure. PLL clock error or jitter plays an especially important role when dealing with high speed communication between ASIC chips. Therefore, accurate knowledge of PLL jitter is critical in avoiding system failures.
Unfortunately, the ability to accurately measure output clock error has become increasingly difficult because production testers used for ASIC tests cannot accurately measure jitter if the PLL errors are reduced to a point. Furthermore, even though certain PLL parameters can be measured and tested, there exists no guarantee that a PLL could still exhibit jitter above its specification.
FIG. 1 shows an IC having PLL jitter measurement in the related art. With reference to FIG. 1, an integrated circuit 10 has a first PLL 11 and a second PLL 12. The first PLL 11 generates a reception clock signal rclk for sampling an input signal. The second PLL 12 generates a transmission clock signal tclk for synchronizing with an output signal. A first clock pulse CK1 and a second clock pulse CK2 are sent from the outside of the integrated circuit 10 to the first PLL 11 and the second PLL 12, respectively. The reception clock signal rclk is distributed to a functional circuit such as a logic circuit (not shown) of the integrated circuit 10. The transmission clock signal tclk is used to send transmission data of a transmission functional circuit 18 that has a transmission function to the outside of the integrated circuit 10.
The integrated circuit 10 has a PLL jitter measurement functional circuit 13 that measures a jitter of the reception clock signal rclk. The PLL jitter measurement functional circuit 13 receives transmission data from the transmission functional circuit 18. The PLL jitter measurement functional circuit 13 selectively transmits reception data in synchronization with the reception clock signal rclk or transmission data received from the transmission functional circuit 18 in synchronization with the transmission clock signal tclk.
The PLL jitter measurement functional circuit 13 has a sampling portion 14, a delaying circuit 15, a selecting circuit 16, and an outputting portion 17. The sampling portion 14 samples the reception data (as an input signal IN) with the reception clock signal rclk, and obtains a sampled signal. The delaying circuit 15 delays the sampled signal, and sends the delayed sampled signal to the selecting circuit 16. In the jitter measuring mode, the selecting circuit 16 outputs the delayed sampled signal to the outputting portion 17. In the normal mode, the selecting circuit 16 outputs the transmission data received from the transmission functional circuit 18 to the outputting portion 17. The outputting portion 17 outputs the transmission data as an output signal OUT in synchronization with the transmission clock signal tclk.
When a conventional IC tester 19 is connected to the test input terminal and the test output terminal of the IC 10, the PLL jitter measurement functional circuit 13 is enabled to measure jitter parameters of the clock signals rclk and tclk. The clock signals rclk and tclk are sampled and the number of the expectation errors are counted to determine the jitters.
Referring now to FIG. 2, another conventional jitter measurement circuit 20 is shown. To begin jitter measurement, the reset signal 24 is pull low to allow the reference clock 22 and the measured clock signal 26 to the applied to the jitter measurement circuit 20. The jitter measurement circuit 20 measures the jitter defined as the delay between the transition edge of the reference clock 22 and the measured clock signal 26. The jitter measurement circuit 20 functions by sorting transition edges of the measured clock signal 26 into one of several possible time slices wherein each time slice represents a different time lag of the measured clock signal 26 behind the reference clock 22.
The jitter measurement circuit 20 includes a delay stage 21; a latch stage 23; and a four-bit word output 28. The delay stage 21 delays the reference clock 22 into clock signals 22a˜22d. The latches in the latch stage 23 captures value of the measured clock 26 (either 1 or 0) at a specified transition edge of one of the various delay clock signals and outputs the captured value over a corresponding output line OUTA, OUTB, OUTC and OUTD. In other words, the latch stage 23 captures the value of the measured clock at a specific point in time to determine whether the transition edge occurred. If the captured value is 0, then the transition edge of the measured signal has yet to occur. If the captured value is 1, then the transition edge of the measured signal has already occurred. Thus, a four bit word 28 is generated that contains information regarding when a rising transition edge of the measured clock occurred. The four bit word 28 indicates the amount of jitter between the rising edge of the reference and measured clocks.
Therefore, it needs a new structure jitter measurement circuit which may provide advantages over the related arts.